1.[Purpose of Research] : In case of using the medical X-ray apparatus, the measurement of X-ray effective energy can not carry out simply, because X-ray attenuation curve must be measured troublesome. Then, We considered a method which X-ray effective energy can be measured very simply by using a concept of Quality Index (QI). QI showing the distribution width of continuous X-ray spectrum is expressed by E_<eff> /E_<max>, where E_<max> is maximum energy of spectrum and E_<eff> is effective energy. From E_<max> is equal to value of X-ray tube voltage (kV), if QI values are prepared against various relating factors, X-ray effective energy can be determined easy by multipling QI by kV. In this research, many data of QI were produced for primary X-ray and scattered X-ray.2.[QI measurement of primary X-ray] : The QI value depend on many X-ray quality factor, but in this research, QI measurements were carried out to following factor. (1) X-ray tube volt-age (50-150kV, 10kV step), (2) X-ray
… Moretube current (1, 100, 300mA), (3) high voltage wave form (single phase 2 peak type, inverter type, three phase 12 peak type, constant voltage type).3.[QI estimation by spectrum] : The QI value for scattered X-ray can not be measured directry, because the measurement of attenuation curve for scattered X-ray are impossible. Then, a method which QI value is estimated by calculating from a scattered X-ray spectrum was investigated. And the spectrum were measured by a Ge-semiconductor detector. As result, both QI values of the expriment and the calculation agreed with high accuracy. Under this results, QI value for scattered X-ray were determined by the calculation using spectrum.4.[QI estimation of scattered X-ray] : Using Ge-detector, the spectrum for scattered X-ray were measured against following factor. (1) X-ray tube voltage of primary X-ray (50-120kV , 10kV step), (2) volume of scattering material(1^3, 5^3, 20^3 cm^3 , Water and Acryl.), (3) scattering angle (30ﾟ- 150ﾟ, 30ﾟ step). The measured spectrums were converted to QI value by calculating respectively. Less